NUCLEAR FISSIONDOE-HDBK-1019/1-93Atomic and Nuclear PhysicsAs the atomic number and the atomic mass number increase, the repulsive electrostatic forceswithin the nucleus increase due to the greater number of protons in the heavy elements. Toovercome this increased repulsion, the proportion of neutrons in the nucleus must increase tomaintain stability. This increase in the neutron-to-proton ratio only partially compensates for thegrowing proton-proton repulsive force in the heavier, naturally occurring elements. Because therepulsive forces are increasing, less energy must be supplied, on the average, to remove a nucleonfrom the nucleus. The BE/A has decreased. The BE/A of a nucleus is an indication of its degreeof stability. Generally, the more stable nuclides have higher BE/A than the less stable ones. Theincrease in the BE/A as the atomic mass number decreases from 260 to 60 is the primary reasonfor the energy liberation in the fission process. In addition, the increase in the BE/A as the atomicmass number increases from 1 to 60 is the reason for the energy liberation in the fusion process,which is the opposite reaction of fission.The heaviest nuclei require only a small distortion from a spherical shape (small energy addition)for the relatively large coulomb forces forcing the two halves of the nucleus apart to overcomethe attractive nuclear forces holding the two halves together. Consequently, the heaviest nucleiare easily fissionable compared to lighter nuclei.SummaryThe important information in this chapter is summarized on the following page.NP-01Page 54Rev. 0